Convective Structures in Clear-Air Echoes seen by a Weather Radar

Transcription

1 Convective Structures in Clear-Air Echoes seen by a Weather Radar Martin Hagen Deutsches Zentrum für Luft- und Raumfahrt Oberpfaffenhofen, Germany

2 Weather Radar Weather radar are normally used to locate precipitation estimate rainfall rate apply nowcasting Advanced radar systems now implemented in Europe, use Doppler and polarization capabilities for improved retrieval algorithms for rain rate estimation and nowcasting. 2

3 Some Radar Basics The radar measures the power of a transmitted electro-magnetic wave packet reflected by a particle: P t P r r Radar equation for volume targets: P r = Radar constant P t g 0 h ln (2) r Vol Particles smaller than the wave length: (C-Band = 5 cm, D < 5 mm) Rayleigh-scatter 2 i Reflectivity (sum of the scattering cross-sections) 5 = 2 6 i K 4 D i Reflectivity z: Unit: mm 6 m -3 logarithmic unit: dbz 3

4 Radar Reflectivity dbz rain mm/h Interpretation typical sensitivity: -5 dbz at 50 km range 0 dbz at 100 km range hail severe rain, hail strong rain, graupel rain cumulus hum. -30 dbz light rain drizzle?1 drop 1 mm per m³ 1 drop 0.65 mm per m³ "clear-air" "clear-air" 4

5 Clear-Air Echoes? radar echoes from precipitation and cloud free atmosphere (cumulus clouds give Z -30 dbz) not ground clutter!! what else?? Radar Satellite 5

6 Clear-Air Echoes Radar meteorologists know two sources for clear-air echoes: Bragg scatter strong gradients in refractive index on a scale of ½ λ (stronger at longer wavelength λ -> wind profilers) need of warm humid air with strong turbulence, possibility to be achieved in summer for C or S band small particles like wet large aerosols aerosols are small, would need a very large number insects, birds large (2-4 mm) would need only a few in measurement volume 6

7 Clear-Air Echoes (horizontal display 40 km range) Doppler Velocity Reflectivity 7

8 Clear-Air Echoes (vertical cross section) Doppler Velocity Reflectivity 8

9 Lidar observations for same event Lidar observations from aircraft (downward looking) 9

10 Clear-Air Echoes similar observations of vertical structure seen by radar, lidar and radio sonde must be particles! must be related to vertical humidity/temperature layers! insects!!?? - are particles - present within the convective boundary layer other evidence for insects: polarimetric signature 10

11 Polarimetric Weather Radar and Insects Polarimetric weather radar can measure differential reflectivity (ZDR) as a measure of horizontal to vertical axis ratio. Mainly used for rain rate estimation and hydrometeor identification: Rain: ZDR 0 5 db b a Insects ZDR 5 10 db a b Z ZDR 11

12 ZDR Radar and Insects Insects act as passive tracer, small insects do not have the power to fly against wind or convection try to avoid humid regions (clouds, sea) prefer to see where they are going Doppler velocity wind

13 Convective Structures visualized by Insects vertical plumes Reflectivity 13

14 Convective Structures visualized by Insects horizontal alignment of convection wind 14

15 Convective Structures visualized by Insects Reflectivity Doppler velocity wind 15

16 Clear-air Echoes can we see structures as simulated by Schmidt, 1988? (Schmidt, 1988) WMO Manual,

17 Clear-Air Echoes 13 July 2007 (COPS campaign) Dual-Doppler analysis to get horizontal wind field (to be done). Reflectivity (DOW) Doppler velocity (DOW) Doppler velocity (POLDIRAD) 17

18 Summary and Conclusion Clear-air echoes from weather radar can give insight in the spatial structure of convection Clear-air echoes can be observed throughout the summer time mainly within the boundary layer Access to raw data needed, because of reflectivity is well below rain values normally provided by weather services Validation by IGC files? DWD Feldberg radar Doppler velocity 18